Breeze Darker is a GTK theme to match my Breeze Darker Plasma color scheme.
I have only used this theme on KDE Plasma and can confirm it works great with my Breeze Darker Plasma color-scheme.
Wallpapers are from Charlie Henson:
Set look and feel to Breeze
Install color scheme
Install Breeze-Darker GTK theme
Set the themes. You will have to select Breeze Darker in Kate
Lots of screenshots here: